This invention relates to hydraulic reaction turbines; and, more particularly, to a system and method for reducing the effects of draft tube pressure fluctuations acting on a runner in a hydraulic reaction turbine.
Hydraulic reaction turbines have been known in the art for many years. In the early development of such turbines, it was discovered that turbines which were operated at heads and flows other than the design point of the turbine, experienced rough operation and power swings resulting from water passing out through the draft tube of the turbine in a whirling and spiraling vortex. This is particularly true of Francis-type turbines, propeller turbines and pump turbines. In April 1940, W. J. Rheingans published a paper in the Transactions of the American Society of Mechanical Engineers in which he outlined the results of an extensive study of the draft tube surging phenomenon in turbines. In his paper, and in the comments from others, several schemes for suppressing draft tube surging were discussed; these included fins projecting from the draft tube walls, an extended runner cone filling the space occupied by the draft tube vortex, venting of the turbine head cover and air injection into the draft tube through the straightening fins and through a hole in the draft tube wall. Since that time there have been numerous papers on the subject and many suggested schemes to reduce draft tube surging. Such schemes have included air admission to the turbine runner, to the draft tube, to the lower side of the runner through the runner cone, or through holes in streamlined shapes placed across the draft tube. Air has also been injected into the penstock, into the space between the wicket gates and the runner, and through the runner band. There have also been many schemes for the use of fins in the draft tube; these include flow splitters and a coaxial hollow cylinder placed below the center of the runner cone and supported by fins which can be rotated, and several types of fins as discussed hereinabove.
When a hydraulic reaction turbine is operated at a head and/or flow which is different from the design point, the water will follow a helical path as it passes out through the draft tube. If the operating conditions are sufficiently remote from the design point, a vortex will form in the swirling water just below the runner cone and this vortex usually will follow a helical path as it passes out through the draft tube. The core of the vortex is usually filled with water vapor. The spiraling vortex causes pressure fluctuations which vary the net head experienced by the runner and cause the generated power to fluctuate in a similar manner. Net head is also affected when the water vapor bubbles in the core of the vortex collapse.
In hydro facilities that use reaction style turbine runners, there is a hydrodynamic instability that occurs at partial load. This instability is often referred to as the rough load zone and corresponds with Rheingans influence discussed above. This instability typically occurs when the turbine-generator is operated in a region of 25% to 75% of the rated machine power output capacity, as the rough load zone varies for each machine. The result of this instability is high vibration of the machine components, with the possibility of damage or premature wear to various machine components. The frequency of this vibration is typically between 0.15 and 0.35 times the running speed of the machine.
In order to minimize this vibration, air is injected into the flow of water through the turbine. The accepted industry practice is either to provide air injection constantly during operation or based on generator load. However, operation of compressors to inject air is an additional expense which reduces plant profits.
Thus, there is a need to enable operators of hydro facilities using reaction style turbine runners to engage air injection equipment to reduce vibration due to hydraulic effects at partial load and engage such equipment only when necessary due to vibration. There is also a desire to minimize the costs associated with air compression while protecting the hydro machine from damage or excessive wear due to vibration caused by the instability.